Significant Severe Weather Outbreak Possible Friday Night and Saturday

FORECAST:

Thursday (High 72, Low 53): Rain showers likely off and on throughout the day. Isolated thunderstorms are possible and could produce hail and perhaps gusty winds. 

Friday (High 78, Low 55): Partly to mostly cloudy and windy during the day. Thunderstorms are likely at night, and some storms will likely be severe. 

Saturday (High 73, Low 63): Thunderstorms likely - possibly severe. The peak time for severe weather is in the afternoon/evening hours and will include a threat for tornadoes as well as large hail and damaging thunderstorm winds. 

EXTENDED OUTLOOK:

Sunday (High 64, Low 51): Mostly sunny. 

Monday (High 65, Low 36): Sunny.

Tuesday (High 71, Low 39): Sunny.

Wednesday (High 72, Low 48): Partly cloudy with a 20% chance of showers/thunderstorms. 

PRONÓSTICO:

Jueves (Máxima 72, Mínima 53): Probabilidad de lluvias intermitentes durante el día. Posibles tormentas eléctricas aisladas que podrían producir granizo y quizás ráfagas de viento.

Viernes (Máxima 78, Mínima 55): Parcialmente a mayormente nublado y ventoso durante el día. Probabilidad de tormentas eléctricas por la noche, y algunas tormentas probablemente serán severas.

Sábado (Máxima 73, Mínima 63): Probabilidad de tormentas eléctricas, posiblemente severas. El pico de tiempo severo es por la tarde/noche e incluirá amenaza de tornados, así como granizo grande y vientos de tormenta eléctrica dañinos.

PERSPECTIVA EXTENDIDA:

Domingo (Máxima 64, Mínima 51): Mayormente soleado.

Lunes (Máxima 65, Mínima 36): Soleado.

Martes (Máxima 71, Mínima 39): Soleado.

Miércoles (Máxima 72, Mínima 48): Parcialmente nublado con un 20 % de probabilidad de lluvias o tormentas eléctricas.

NOTES:

Review your tornado safety plan before we have any problems Friday night or Saturday. This could be really important with this event, to have a reliable way to get alerts and a clear safety plan. 

We have a lunar eclipse tomorrow night/Friday morning, also known as a "blood moon", and some of us might actually be able to see it, depending on how the clouds break up or not. 

And there are still a few SKYWARN classes available between now through the month of April, if you'd like to be trained as a storm spotter. It's free, and it's fun. 

DISCUSSION:

It was a sunny day in Cullman with a southerly breeze. The High was 75, and the Low was 45. 

As of 7:35 PM CDT we have clear skies and 64 degrees. The dewpoint is 36 degrees, making the relative humidity 34%. Winds are from the Southwest at 6 miles per hour. The pressure is 29.88 inches/1011.9 millibars and rising slowly. Visibility is 10 miles. 

It was a lovely day before we get into some days of mostly rain tomorrow and then some serious stormy weather this weekend. The thing this weekend may be a significant severe weather event that includes tornadoes. So we'll look at it in detail. 

As of right now, our High pressure system is shifting from the Gulf into Atlantic, you can see the two Hi circles on both sides of Florida's Peninsula there. And out in the Central Plains, we have a Low pressure system and associated front that stretches up through the Ohio Valley. 

Our upper-level winds are generally zonal from the West, winds at the surface generally from the Southwest. 

Thanks to the folks at WeatherNerds for providing backup satellite imagery when the official NOAA/GOES site is experiencing a lag in data like tonight. 

The only rain that appears to be actually reaching the ground on the radar right now is way out in Eastern Texas. 

And as I look at that satellite image, it still looks delayed. So I'm going to try again. 

And the official GOES imagery may be over an hour old, flagged that way on their site, but it was better than the animation I got from WeatherNerds this time, great as that site can be. You can actually see the lightning from those storms in East Texas on this image. And it's good to start with a four-dimensional view of the atmosphere: Radar, Satellite, Upper-level Maps (well I show one anyway, occasionally show more, but for today using the most commonly referenced at 500 millibars or 18,000 feet), and Surface Maps. 

And now let's take a look at the results of the latest upper-air soundings from this evening. 

I've had such a hard time getting these Skew-T graphics to show up properly since I started trying to make this a habit that tonight I just screenshotted them. The air is stable for now, and the wind fields are just typical for a Spring day when we're in between rain-or-storm-making systems. But these are for Birmingham, Nashville, and Jackson (MS). They were taken at 7 PM CDT/00Z Universal Time. We have these all around the country (although I've heard of three sites suspending launches due to budget cuts and short-staffing . . . we try to deal with such bumps in the road as they come along), and it's a big part of the data that is fed into all these awesome computer models we have now, that use complex equations (like differential equations and calculus) to forecast the future state of the atmosphere for up to 7-10 days or even beyond. I only trust them for about 7-10 days except for a clue to general trends. And even in days 7-10, sometimes it's general trends. But it's a lot better than what we used to have even 20-30 years ago. And I've decided to start trying to remember to show the local upper-air data on these Skew-T diagrams every day so that anybody who reads this blog understands that they help us (and the computer forecast models) make a proper diagnosis of the atmosphere better than we could without them. They launch these radiosonde devices in balloons twice a day, every day. During severe weather, sometimes they make a special launch to get a look at every level of the atmosphere and analyze it carefully. Human forecasters need to do that, not just relying on computers. 

Here's a broad view of how the weather systems will be moving between now and Friday night. Rain returns to our forecast tomorrow, and then we'll have a period of clearing and warming up before severe thunderstorms are possible Friday night, especially over our Western counties but really the risk is there for all of us. We'll get into those details below. Just a lot of people like to see traditional weather maps along with raw model output, which can be harder to read if you're not used to it. 

And sometimes I forget that kinda' stuff. My brothers both consider themselves more sophisticated and intelligent than I am. About some things, they are, but I can't say I've ever cared for that assessment, and don't think it's true about everything. And the more pompous of the two said to me a long time ago, seeming rather embarrassed to admit it, that he couldn't really get through a lot of the technical weather jargon I was using. He was the one who convinced me to start putting weather forecasts up at the top, the short, simple version, and putting the detailed discussion at the bottom. I used to write out discussions (years ago on a different blog) and then put a bullet-points forecast at the very bottom. And he helped me see I was losing my audience before they ever got to the summarized forecast. 

So anyway here we are. 

Tomorrow we have a shortwave trough moving in with some rain. And we could see some isolated thunderstorms in the mix, just enough unstable air for that. We'll have a southerly breeze at times, and we should see a High temperature of about 71-72, a Low of about 52-53. 

The Storm Prediction Center has outlooked a Marginal Level 1 Risk of severe thunderstorms for tomorrow, mainly for Central and South Alabama, clipping parts of Mississippi and Georgia. I've looked at the forecast parameters over the last few days, and I'm not very impressed with this outlook. It's one of those rare times I'll disagree with the SPC and think they are being overly vigilant. I'm afraid this may take some people's attention away from the really important long-duration event Friday night and Saturday. But I'm passing it along because if I don't, people will still see it from other sources and wonder why I'm not mentioning the severe thunderstorm threat for tomorrow. 

It does technically include counties like Cullman, Walker, Winston, even up to Franklin County in Northwest Alabama. 

In my opinion, this is a very low chance for one or two storms that may produce marginally severe hail (up to about the size of quarters), and I guess we could see a damaging wind gust of up to about 60 mph with this somewhere. But the chance is so low that I started not to even show it. This is the lowest risk level they issue, and they've made it clear that they are not worried about a tornado threat with this, just a very low risk for severe hail and/or winds. 

But I think most of us are just going to get periods of rain tomorrow, a lot of places probably not even having enough unstable air for a whole lot of thunder. But there will be thunderstorms in the mix, and if one or two were to approach severe limits, you've had a heads-up. Usually I defer to the judgement of the SPC since they do forecast severe weather for the whole country every single day. Even by their graphics and description, this risk is about as low as it gets. And I think most places will just see rain with a lower risk for thunderstorms. If there is hail in the storms, in my personal opinion, even most of that or any wind gusts will stay below severe limits. 

Then the serious business starts Friday with a deep Low pressure system and negative-tilt trough moving through the Central and Northern Plains. Now around here, during the day, skies should stay partly cloudy, increasing clouds toward evening, and the winds will be picking up, with gusts up to 20 mph or greater. Our High temperature should be about 77-78 with a Low near 55. 

It is at night that we'll have to start watching for severe weather around here. About 9 PM is a good time to estimate, we'll have to start watching things closely. And you can see, this looks like a widespread outbreak of severe weather Friday into Friday night throughout many states. That hatched area is the zone where supercell thunderstorms are most likely to form and track along. 

Since I criticized them above (forecasting this stuff can be a headache, and sometimes I get a little grouchy), I'll say that I think the SPC is totally nailing it with this outlook, as they usually do. I fully agree and have high confidence in this outlook for Friday night around here. 

And that hatched area that is the hot zone for supercells is from Huntsville, Cullman, Fayetteville, and points West. With this kind of setup, everybody should be prepared for a severe thunderstorm capable of producing a tornado. But sometimes it's good to know the threat levels for different areas just as a basic guideline. 

So that's our first round of organized severe weather, a pretty wide window from roughly 9 PM Friday night to 7 AM Saturday morning. We could see a mixed storm mode of some supercells and maybe a cluster or line segment. But large hail, damaging winds, or a tornado are possible in any of these storms. I'm not going to sugarcoat it: Everybody needs their NOAA Weather Radio (or at least cell phone with Wireless Emergency Alerts) ready to wake them up Friday night/early Saturday morning in case of severe weather. And everybody, especially people who live in mobile homes, need a clear plan to get to a place of reasonable shelter within five minutes of a Tornado Warning being issued for their county, at least if they fall within the warning polygon. 

And the GFS continues to show that secondary surface Low forming in a favorable position to enhance our severe weather potential around here as this front comes in Saturday. 

The NAM has come on board with this, just has a slower timing. And yesterday the ECMWF, other main global model than GFS, had come on board with the idea. So confidence is high that this will be a stormy day, maybe a very stormy day. We're looking for a High in the lower 70's, a Low near 60 or so. And we could have a midday round of storms somewhere, but overall we're looking at two rounds here that we expect: The first from Friday night into Daybreak or so Saturday morning. Then probably a lull in the action until Saturday afternoon/evening, when we could see even worse storms. The atmosphere will probably be able to recover enough to support supercell thunderstorms before an eventual squall line in the evening/night hours. That's how this is looking. 

Between 1-4 PM Saturday, the Energy-Helicity-Index starts to look really concerning. Now it's worst along and near the border of Mississippi. But the values are pretty high for all of North Alabama up into Southern/Middle Tennessee. 

And a forecast sounding for where the EHI is a 2.4 value in parts of Cullman, Walker, and Winston Counties clearly shows a tornado threat in addition to the chance for damaging winds and large hail. We've got a Supercell Composite of 10.6 and a Significant Tornado Parameter of about 2-3. So we could even have a tornado that stays on the ground a while, doing significant damage. Our CAPE is under 1,000 j/kg but Storm Relative Helicity over 400 units even at the lowest kilometer of the atmosphere. And you really only need that value to be about 150-200 units to support tornado development. If you've got any instability to work with at all. So this is a threatening setup. 

There are times Saturday afternoon when our surface-based CAPE values are expected to exceed 1,000 units, which is moderately unstable around here this time of year. 

And during that same time, it'll be common to see Storm Relative Helicity values down at the lowest 1 kilometer of 200-300 m^s/s^2 or greater. And you really only need about 150 units of that to support tornadoes, even if the instability is weak (which it isn't with this event). So this is a pretty good recipe for tornadoes to be able to develop in our region. 

The Significant Tornado Parameter (which I trust more than the Weather Channel's TORCON, even though I met Greg Forbes one time and really liked the guy) is generally from 1-4 over North Alabama into Southern Tennessee, and then by 4 PM we start to see more values here and there of 5-8. This points to a very significant tornado threat. We definitely are going to have to watch for the potential of some particularly damaging tornadoes in the mix with this system. 

At least that's what the NAM is showing. 

Now sometimes, the NAM has been known to make things look scarier than it really should. But I don't think it is in this case. It is a mesoscale model for North America, good for out to 3-4 days. But the global model, the GFS American model, has shown stuff at least this ominous over the past few days. I think this guidance is pretty accurate, even if we may find it unpleasant. 

The only thing making me take a step back and reconsider is this guidance from the SREF, which even though lots of people say it's outdated, has often proven (to me) to be the most reliable and balanced when other mesoscale models are out to lunch, or the CAM's are out to lunch sometimes. It is showing the best unstable air in place for the overnight round, between about 1-4 AM early Saturday morning. So this is a slower timing than what models have shown for the overnight round of severe weather anyway. 

And the SREF, the SPC's in-house model, is showing the atmosphere not being able to recover enough for any meaningful instability up in North Alabama. This is a discrepancy worth taking a look at. I'm skeptical of it, and I think the NAM (as well as the GFS and ECMWF over several days) is closer to getting this right. But this outlier has to be considered. Having two rounds of severe weather in a row like this, there's always that question of how much the atmosphere can recover and get unstable again. I think we'll have plenty of instability Friday night, and really, all this SREF guidance does for me is make me think it might not get quite as bad as the NAM makes it look. I still think it'll be a significant threat for severe thunderstorms and tornadoes Saturday afternoon/evening. But if the SREF is on to anything here, that might keep it from being as high-end an event as it could otherwise be. 

These are nitpicky details, and I'd advise everyone to err on the side of caution here and prepare for an outbreak of severe thunderstorms capable of producing tornadoes Saturday afternoon and evening. 

All of us in North Alabama and Southern Middle Tennessee have a risk for organized severe weather on Saturday, and while the most likely timing is in the afternoon and evening, really any time of day is fair game. And yes, the chance may be greater in that 30% enhanced risk area, but really anybody could get severe weather from this in this broad area, including a more significant event, like some extra-large hail, really destructive damaging winds, or a tornado that stays on the ground a while and does significant damage. 

This threat is very real, and everybody in this broad zone needs to respect it and have a good way to get alerts and a clear safety plan that everybody in the family (or any friends visiting, or co-workers, lots of people have to work Saturdays) knows - what you'll do if a Tornado Warning is required. 

Then on Sunday that severe thunderstorm risk shifts to Georgia and the Atlantic Coast states, all the way up to Pennsylvania and as far South as parts of the Florida Panhandle and perhaps the Bend. 

Here's a regular weather-map view of the cold front pushing through our area Saturday and then bringing us another shot of cool air behind it Sunday. 

It looks like we'll clear out rapidly Sunday with sunny skies and a High in the lower, maybe mid-60's, the Low about 50 or so. 

Then we'll have another High pressure system moving through on Monday and Tuesday, sharp contrast to the weather we had on Friday night/Saturday. That's how spring-time is around here lots of times. 

Monday looks sunny with a High in the lower 60's and Low in the upper 30's. Tuesday staying sunny with a High of about 70 or so and a Low near 40, although with such dry air, radiational cooling, we may still be looking at upper 30's. Once again I'm second-guessing the computer model guidance, looking at the National Blend of Models and adjusting down a couple degrees maybe. 

Then it's questionable how quickly the next system affects us on Wednesday, so only going to put a 20% chance of rain back in the forecast and looking for a High in the lower 70's, Low in the upper 40's. 

We'll have to watch for scattered instances of flash flooding with this storm system over the weekend, but the main issue is severe thunderstorms capable of producing tornadoes. You can always see localized flooding from supercell thunderstorms. They tend to dump a lot of rain as well as hail. 

On average, rainfall totals should be more in the 2-3 inch range over our region. And we do need this rain. 

Buy Me a Coffee

Looking at the analog events that came from a similar atmospheric setup, the two that are showing up the most often and getting my attention are the January 30, 2013 event and the infamous tornado outbreak of April 8, 1998. And then there are a lot of analogs that produced very little severe weather in the region or just an average sort of severe weather event that included a few tornadoes along with damaging winds and large hail. But those two I mentioned are currently showing up for both Friday and Saturday. And they have shown up repeatedly over the past several days. I think those two events represent the spectrum of what we could see Friday night/Saturday. We might just get some routine severe weather, but we also have the chance of getting something really significant and extra-damaging, extra dangerous. I notice that only two other tornadoes were reported locally from the April 1998 event besides the F-5 that hit near Birmingham. One of those was an F-2, and the other was an F-3. But even with only three tornadoes out of the whole event, it was a really bad event. And even if we were to luck out somehow (I don't think we will, but I guess one can always hope) and only get an event closer to what the January 2013 event did around here, any tornado is dangerous. Even if it ends up being rated E/F-0, or F-1, you would not want to be caught without shelter, or in something like a mobile home or a vehicle when the tornado hits. If we get more significant tornado damage (a good possibility with this setup), that's all the more reason to take the best precautions you can within your means. 

And by the way, don't let not having an ideal, perfect plan stop you from putting into action a plan that is probably going to be good enough. Even if you'll be driving, just plan at least 3 or 4 places along your route where you could pull over and get into a building, away from windows and preferably in a small room like a bathroom, if a Tornado Warning was issued. 

Let people know this risk is there, and hope everybody stays safe. I don't want to worry anybody to the point of stomach ulcers or whatever, but this risk does look significant. This could be a full-blown tornado outbreak that you have to respect.